In a non-LEO context, I don't think "back-up" is the operative term. A private citizen is not likely to shoot a weapon empty, BUT there are other areas of concern, such as access. Once one gets away from belt carry in a vertical rig, offside-hand access becomes a problem, and size-large folks, or those with shoulder problems, will often have a very difficult time reaching across the body to access a vertical rig.
Attacks usually happen fast, and unexpectedly, and the hand that is nearest the attacker is likely to be busy doing empty-hand defense. This does NOT mean a secondary weapon is a requirement, but it does mean some thought and realistic training is a very good idea.
On another forum, member Anthony coined the term "roving" gun, for a handgun that is worn/carried in a place other than one's primary handgun. It can be carried in different places, depending upon the circumstances, and in some circumstances, will become the first gun for which one will reach. It can be an equivalent, smaller, or larger gun than the primary.
I am an LEO, and due to a specific threat profile for my area at this time, tend to carry two handguns virtually always, when away from the house. Most often, this is two P229 pistols, and sometimes a P229 and an SP101. When I retire, I think I will maintain the habit of two handguns, but downgrade the daily size and capacity a bit, possibly reverting to two SP101 snubbies, which for a while, in the past, was my daily carry ensemble. It is EASY to carry two or even three snubbies, even on my skinny frame. One in a pocket, and one holstered on the "primary" hip, can cover a lot of eventualities.
